1. Field of the Invention
The present invention relates to the prosthetic limbs worn by upper or lower limb amputees and in particular to the portion of a limb prosthesis that is in direct contact with the user's skin.
2. Description of the Related Art
Definitions of Terms
Socket—is that part of a prosthesis in direct contact with the user's skin. The word Socket usually implies a traditional socket that is essentially circular in cross section. A traditional prosthesis consists of an inner socket to interface with the user's skin and an outer socket over it that continues to incorporate the mechanisms that comprise the next distal structure which may be a joint or a device to function as a foot or gripping device. The inner and outer sockets may be separate structures or may be unitary consisting of a single unit.
Interface—is often used as a synonym for socket, but is more often reserved for socket-like structures that have openings in the outer socket and occasionally in both the outer and inner sockets.
Cast—is a thin layer of wet plaster impregnated gauze wrapped around a residual limb and the surrounding body parts and then permitted to harden to reproduce the shape of the limb. While the plaster is hardening, pressure from the hands of the plaster technician often modifies the shape to accommodate the underlying boney anatomy.
Positive Model—is the plaster model that results from filling a cast with plaster or similar material. Modifications by adding and subtracting plaster are made to this model before its outer surface is used to define the shape of the user's socket or interface.
Check Socket—is a temporary socket made over the model and used to test whether the modifications have had the desired effect on the fit of the resulting socket.
Channel—is used here to describe a longitudinal area where the wall of a socket is depressed inward as close to the underlying skeletal structures as is comfortable.
Relief Area—is the region in a socket system between two channels or around or near a compressed area which provides a place for the displaced tissue to migrate.
Lost Motion—is the motion of the skeletal structures with respect to the prosthetic interface when force is applied between the two as would occur as an amputee tries to move the prosthesis as a whole. In a traditional socket lost motion occurs when the bone moves toward the socket wall a substantial distance before imparting force to the wall.
Compression Bar—is a long flat bar typically a little shorter than the shaft of the remaining long bone(s). The width of the bar is usually about ten percent of the circumference of the remaining limb.
Optimal Tissue Compression—is compression of the tissue against the socket wall such that lost motion is minimized without causing discomfort to the user.
High-fidelity Interface or device—is the name given to the socket or interface that utilizes compression stabilization as the basis for its function and physical structure.
Historically the prosthetic user interface has been a cylindrical socket that merely surrounds the remaining limb part with some contouring of the proximal brim so that it will accommodate the shape of the next proximal joint or body part. Typically this socket is made by taking a plaster cast over the limb and filling it with plaster to form a positive model of the limb. Minor changes are made to this shape to relieve boney prominences. When this model is used to create a socket by laminating or thermoforming a layer of plastic there over, the resulting socket mainly encapsulates the limb part. Conventionally, no modification of the traditional model is done. This opportunity to specifically enhance the resulting structure's ability to impart desired motion to the complete prosthesis, and to prevent undesired motion from occurring, has been overlooked, even though these are the most important functions of the interface. The traditional encapsulating or closed volume socket merely contains the soft tissue but does little or nothing to prevent lost motion between the socket and the underlying skeletal structure.
Some improvements have been made in the traditional interface. In particular, many technicians replace the fully encapsulating outer socket with a frame having one or more openings. This change is accompanied by making the inner socket of a flexible material. The resulting frame-style design usually is more comfortable. New materials such as carbon fiber composites add rigidity where needed especially in open frame designs. New flexible materials allow the socket wall to flex in other areas for comfort. Even when these newer flexible materials are used, the soft liner still fully encapsulates the remaining limb as traditionally done, and thus provides a compressive or elastic force to all of the limb's soft tissue.
Conventional laminations over a plaster model work best when the surfaces of the model are convex facing outward, following the general contours of the outside surface of the limb.